The subject matter of the present invention pertains to means for correcting color-pulling or video-pulling in a color display system using a beam-index color cathode-ray tube. A detailed description of beam-index tubes and the color-pulling phenomenon may be found in A. M. Morrell et at., "Color Television Picture Tubes", Academic Press, New York, 1974, all pertinent parts of which are incorporated herein by this reference.
As is known to the art, a beam-index cathode-ray tube includes a specialized display medium comprising a first spaced array of vertically oriented color phosphor bands disposed in a horizontal direction across the interior surface of a supportive faceplate, and a second spaced array of vertically oriented index phosphor bands similarly disposed so as to have a predetermined periodic relationship with the color bands. An electron beam produced within the tube and caused to sweep across the display medium while its current density is suitably modulated selectively excites both the color bands and the index bands to produce not only a desired color image but also a frequency-modulated index signal, the phase of which is a function of the location of the beam relative to the index bands at any given time. The index signal thus produced is employed, in a closed-loop manner, to control the selection of the image-defining color drive signals used to modulate the current density of the electron beam producing both the signal and the image. Such an arrangement ensures ideally that the information being supplied to the electron beam at any given time corresponds precisely to the information desired to be presented at the location defined by the phosphor band currently being addressed by the beam.
A common problem encountered in the operation of beam-index tubes is that of cross-modulation of the index signal by the color drive signals used to modulate the beam current. Absent modulation, the current density of the electron beam is maintained at a level just sufficient to excite the index bands and produce the index signal. The resultant index signal is substantially uniform in frequency and represents accurately the actual locations of the index bands, and thereby the actual locations of the similarly arrayed color bands. However, when the current density of the beam is modulated with the color drive signals to produce a desired image, the sensed index signal tends to lead or lag its true time position and thereby produce an inaccurate indication of the beam position relative to the phosphor arrays. The amount of such lead or lag is dependent primarily on the spaced relationship between the index bands and the color bands, the magnitude of the beam density modulation, the particular color band currently being addressed, and the beam spot size and shape. Such a phenomenon is known as color-pulling or video-pulling. If not corrected, it has a distorting effect on the color quality of the image being produced.
Known attempts to minimize or eliminate the effects of such cross-modulation of the index signal by the color drive signals include the use of separate electron beams to produce the index signal and the color image (the so-called Apple Tube), the use of a non-integral relationship between the pitch of the index bands and the color bands, and distortion averaging (so-called Turner Tube). These attempts, however, have either added undue complexity to the overall system or been ineffective in providing correction for a general class of video drive signals.